The present invention relates to a cathode for an electron tube, and, more particularly, to a cathode for an electron tube having an increased lifetime to emitting a large quantity of electrons for a long time in a high current density area.
FIG. 1 is a schematic section view of a conventional cathode for an electron tube, having a disk-like base metal 2, a cylindrical sleeve 3 for rigidly supporting the base metal 2 from the bottom thereof, a heater 4 placed in the cylindrical sleeve 3 as a heat a source for heating the cathode, and an electron emitting material layer 1 coating the base metal 2.
The electron emitting material layer 1 is generally composed of an alkaline earth metal oxide having barium oxide as a main component, preferably a ternary metal oxide represented by (Ba, Sr, Ca)O.
Such an electron emitting material layer is formed as follows. First, a mixed powder of barium carbonate, strontium carbonate, and calcium carbonate is dissolved in an organic solvent to form a solution. Then, the solution is applied to on the base metal 2 by a process such as spraying or electro-deposition, to form a carbonate salt layer. Thereafter, the electron gun which the electron tube cathode is fixed is mounted in an electron tube and the carbonate salt layer is heated to about 1000.degree. C. by means of a heater, during evaluation of the electron tube. At this time, the carbonate salt is turned into an oxide. For example, barium carbonate is turned into barium oxide as in the following reaction (1). For reference, the name "oxide cathode" is derived because a carbonate salt is changed into an oxide by heating the same at a high temperature during evacuation of the electron tube. EQU BaCO.sub.3 .fwdarw.BaO+CO.sub.2 .uparw. (1)
The generated BaO reacts with a reducing agent Si or Mg, contained in the base metal 2 at the interface between the base metal 2 and the electron emitting material layer 1 during operation of the cathode, and is reduced to free barium, as in the following reactions (2) and (3). EQU BaO+Mg.fwdarw.MgO+Ba.uparw. (2) EQU 4BaO+Si.fwdarw.Ba.sub.2 SiO.sub.4 +2Ba.uparw. (3)
The thus formed free barium is an electron emitter. However, in this process, MgO and Ba.sub.2 SiO.sub.4 are generated and these materials form an intermediate layer at the interface between the electron emitting material layer 1 and the base metal 2. The intermediate layer acts as a barrier that interferes with diffusion of Mg or Si. Accordingly, it is difficult to generate free barium contributing to electron emission, which leads to undesirably reduced life time of the oxide cathode. Also, the intermediate layer has high resistance and prevents the flow of current for emission of electrons, which limits the current density.
According to a recent trend toward higher definition and larger screen display devices employing a cathode ray tube (CRT) devices, there has been an increasing need for a cathode having a high-current density and long lifetime. However, the conventional oxide cathode cannot fill this need, due to disadvantages with respect to performance and lifetime.